FreeBSD Manual Pages

MLOCK(2) FreeBSD System Calls Manual MLOCK(2)
NAMEmlock, munlock -- lock (unlock) physical pages in memory
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS#include <sys/mman.h>
intmlock(constvoid*addr, size_tlen);
intmunlock(constvoid*addr, size_tlen);
DESCRIPTION
The mlock() system call locks into memory the physical pages associated
with the virtual address range starting at addr for len bytes. The
munlock() system call unlocks pages previously locked by one or more
mlock() calls. For both, the addr argument should be aligned to a multi-
ple of the page size. If the len argument is not a multiple of the page
size, it will be rounded up to be so. The entire range must be allo-
cated.
After an mlock() system call, the indicated pages will cause neither a
non-resident page nor address-translation fault until they are unlocked.
They may still cause protection-violation faults or TLB-miss faults on
architectures with software-managed TLBs. The physical pages remain in
memory until all locked mappings for the pages are removed. Multiple
processes may have the same physical pages locked via their own virtual
address mappings. A single process may likewise have pages multiply-
locked via different virtual mappings of the same physical pages.
Unlocking is performed explicitly by munlock() or implicitly by a call to
munmap() which deallocates the unmapped address range. Locked mappings
are not inherited by the child process after a fork(2).
Since physical memory is a potentially scarce resource, processes are
limited in how much they can lock down. The amount of memory that a sin-
gle process can mlock() is limited by both the per-process RLIMIT_MEMLOCK
resource limit and the system-wide ``wired pages'' limit vm.max_wired.
vm.max_wired applies to the system as a whole, so the amount available to
a single process at any given time is the difference between vm.max_wired
and vm.stats.vm.v_wire_count.
If security.bsd.unprivileged_mlock is set to 0 these calls are only
available to the super-user.
RETURN VALUES
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable errno is set to indicate the
error.
If the call succeeds, all pages in the range become locked (unlocked);
otherwise the locked status of all pages in the range remains unchanged.
ERRORS
The mlock() system call will fail if:
[EPERM] security.bsd.unprivileged_mlock is set to 0 and the
caller is not the super-user.
[EINVAL] The address range given wraps around zero.
[EAGAIN] Locking the indicated range would exceed the system
limit for locked memory.
[ENOMEM] Some portion of the indicated address range is not
allocated. There was an error faulting/mapping a
page. Locking the indicated range would exceed the
per-process limit for locked memory.
The munlock() system call will fail if:
[EPERM] security.bsd.unprivileged_mlock is set to 0 and the
caller is not the super-user.
[EINVAL] The address range given wraps around zero.
[ENOMEM] Some or all of the address range specified by the addr
and len arguments does not correspond to valid mapped
pages in the address space of the process.
[ENOMEM] Locking the pages mapped by the specified range would
exceed a limit on the amount of memory that the
process may lock.
SEE ALSOfork(2), mincore(2), minherit(2), mlockall(2), mmap(2), munlockall(2),
munmap(2), setrlimit(2), getpagesize(3)HISTORY
The mlock() and munlock() system calls first appeared in 4.4BSD.
BUGS
Allocating too much wired memory can lead to a memory-allocation deadlock
which requires a reboot to recover from.
The per-process resource limit is a limit on the amount of virtual memory
locked, while the system-wide limit is for the number of locked physical
pages. Hence a process with two distinct locked mappings of the same
physical page counts as 2 pages against the per-process limit and as only
a single page in the system limit.
The per-process resource limit is not currently supported.
FreeBSD Ports 11.2 March 20, 2018 FreeBSD Ports 11.2